Objective lens pole pieces

ABSTRACT

Magnetic lens having pole pieces in which the diameter of the aperture of the upper pole piece and the diameter of the face of the lower pole piece are smaller than the gap between the pole pieces and in which the lower pole piece has at least one frustoconically tapered sidewall.

D United States Patent 13,585,546

I 72] inventors 'laltashl Yanalta; I52! U.S. Cl. 335/210, Kohei Shlrota, both of Tokyo, Japan 250 9 5 (211 App]. No. 868,152 [5 I] Int. CL. H01! 7/00 [22] Filed 0ct.2l, i969 [50] Field ofSearch. ...............2S0/49.5 D; [45] Patented June 15, [971 335/210 [73] Assignee Nilion Denshi Kabushiki Kaisha Tokyo, [56] References Cited [32] Priority Feb. 27, I967 UNITED STATES PATENTS l 2,624,022 12/1952 Wolff 335/210 X 1 41112706 2,819,403 1/1958 Reisner 335/210 x Continuation-impart of application Ser. No. 663,678, Aug. 28, 1967, now Patent No. Hams 3,509,503; Attorney-Webb, Burden, Robinson and Webb ABSTRACT: Magnetic lens having pole pieces in which the diameter of the aperture of the upper pole piece and the diameter of the face of the lower pole piece are smaller than [54] OBJECTIVE LENS POLE PIECES the gap between the pole pieces and in which the lower pole 3 Claims, 6 Drawing Figs. piece has at least one frustoconically tapered sidewall.

I UDFF SI PATENTEUJUNI 51911 SHEET 1 OF 2 INVENTORS.

Ta/rashi Yanaka Kohe/ Shirafa M MM,IW%W

THE/R ATTORNEYS PATENTEU Jum 5m 3565.546

Lens 3 b| 6 D2 2 65 IO 2 60 2 0.5 66 IO 2 60 4 I0 67 IO 2 60 6 L0 O (mm) 4 6 F- 6 J(KA) INVENTORS.

I I9. Ta/rashi Yanaka Kohei Shirota 6) W660 ,W.M I I THE If? A TTORNEYS OBJECTIVE LENS POLE PIECES This invention is a continuation-in-part of our application Ser. No. 663,678, filed Aug. 28, 1967 entitled Objective Lens Pole Pieces now U.S. Pat. No. 3,509,503, which claimed priority under 35 U.S.C. 1 19 to Japanese Pat. application No. 42-12706, filed Feb. 27, 1967.

in conventional: type objective lenses, the gap between the pole piecesand the aperture diameter of these pole pieces are made extremely small. However, by making the gap and the aperture diameter extremely small, the space between the pole pieces is insufficient to meet the requirements for the various attachments which must be inserted therein.

In our copending application, Ser. No. 663,678, referred to above, we disclosed and claimed objective lenses that diminish spherical and chromatic aberrations while providing a large aperture in the upper pole piece. In this specification, we disclose and claim objective lenses that diminish the same aberrations while providing a large gap between the pole pieces. These results were achieved using relatively small amounts of excitation current in the pole pieces.

We have found that both the spherical and chromatic aberrations may be reduced by forming the lower pole piece with a frustoconical shape and by minimizing the diameter of the lower pole face. By so constructing the lower pole piece, the magnetic field of the lens is concentrated immediately at the top of the lower pole piece. The magnetic flux at the face of the lower pole piece increases more rapidly than the flux entering the frustoconical sidewalls of said pole piece, thereby making the field distribution near the top of the lower pole piece similar to the usual symmetrical objective lens pole piece having extremely small gap and aperture diameter. This construction has resulted in minimizing the aberration and focal length utilizing a relatively small excitation current.

According to this invention, a magnetic electron objective lens has upper and lower pole pieces spaced apart by a distance S. The pole pieces have apertures aligned along a common axis. The lower pole piece has a pole face normal to the axis having a diameter D which is equal to or greater than the diameter b, of the aperture of the upper pole piece. The lower pole pieces have sidewalls that have at least one conical taper between 40 and 60 with respect to the axis. The diameter 11, of upper pole piece and the diameter D, of the lower pole face are equal to or less than the distance S between the pole pieces. It is preferable according to this invention to maintain the diameter b of the aperture of the lower pole piece as small as possible and at least smaller than one-half the diameter D of the lower pole face. According to a preferred embodiment of the invention, the gap S is between 4 and 20 mm. wide. The lower pole face D is preferably within 6 mm. in diameter.

In the accompanying drawings, we have shown preferred embodiments of the invention in which:

FIG. 1 is a vertical section through the pole pieces of an objective lens of our construction;

FIG. 2 is a partial section ofthe objective lens shown in FIG. I including an axial field distribution curve between the upper and lower pole pieces;

FIG. 3 is a partial vertical section through another embodiment of our invention;

FIG. 4 is a partial vertical section through still another embodiment of our invention; and

FIGS. 5 and 6 are graphs showing the variations in the chromatic aberration coefficient of the asymmetrical objective lens according to this invention in which the frustoconical surface is 60.

Referring to FIGS. 1, 2 and 3, 1 represents an asymmetrical objective lens pole piece. Upper pole piece 2 is spaced from lower pole piece 3 by means of a nonmagnetic material 4. Pole pieces 2 and 3 have beam passages 7 and 8, respectively, that are coincident with each other. These pole pieces are connected to the end of the objective lens yokes 5 and 6. Lens coil 11, excited by an energizing current, is adapted to excite the pole pieces greater than that required for saturation. The upper pole piece 2 has an aperture diameter b, equal to or smaller than the diameter D, of the lower pole face. Conical surface 9 of the lower pole piece 3 has an angle 0 with respect to the lens axis 10.

The magnetic field distribution located at the gap between upper pole piece 2 and lower pole piece 3 of the asymmetrical I objective lens is shown in FIG. 2. As is apparent from the HQ, the point where the field strength becomes maximum (B,,,,,,.) is

considerably lower than one-half the distance between pole pieces 2 and 3. In the region of saturation of the lens pole pieces, the maximum field and half width d of the image side field distribution are not only varied by increasing the excitation current of the pole pieces, but are also altered by changing the geometry of the pole pieces, such as lower pole face diameter D frustoconical surface angle 0, gap S, and aperture diameters b, and b. I

As explained in our copending application, Ser. No. 663,678, referred to above, the spherical and chromatic aberration coefficient to magnetic lenses can be considerably reduced by increasing the excitation current .I(KA). It is, of course, desirable to minimize excitation current. Further objects and advantages of this invention would be apparent from a study of the following examples.

A magnetic lens 55 according to this invention was provided with an upper pole piece aperture having a diameter b, of 2 mm. The lower pole piece had a 2mm. pole face diameter D a 0.5mm. aperture and a conical sidewall having an angle 0 with the axis of 60. An almost identical lens 56 was provided with a 4mm. lower pole face diameter D FIG. 5 illustrates graphically the chromatic aberration coefficient C, as a function of excitation current J(I(A) for the lens described above. The curves for each lens is marked 55 and 56, respectively. FIG. 5 establishes that decreasing the diameter D decreases the chromatic aberration. Furthermore, the smallest possible aberration coefficient obtainable is the same or less than the coefficient for a lens having the same lower pole geometry D 0 and i b as referred to in FIG. 5 in our copending application, Ser. No. 663,678. It will be noted from the curves in FIG. 5 in this application that with a small excitation current the chromatic aberration coefficient can be reduced to less than lmm.

FIG. 6 illustrates graphically the chromatic aberration coefficient C, as a function of excitation current .I(I(A) for three additional lenses 65, 66 and 67. All three lenses were constructed such that the gap between the pole pieces was l0mm., the diameter of the upper aperture b was 2mm. and the slope of the sidewalls of the lower pole pieces was 60. However, the size of the faces and apertures of the lower pole pieces were varied. In lens 65, D was 2mm. and b was 0.5mm.; in lens 66, D was 4mm. and b was 1mm.; and in lens 67, D was 6mm. and b was lmm. The curves for these lenses are marked 65, 66 and 67. FIGS. 5 and 6 establish that the smallest possible aberration coefficient does not change even when S is changed from 4mm. to l0mm. However, an increase in excitation current is necessary. In other words, the very small aberration coefficient is dependent upon D, and 0; that is to say, the coefficient becomes small when D, is small regardless of the size of S. It is also apparent that, in order to decrease the coefficient when S is constant, the diameter D, should be decreased.

According to this invention, very small aberration coefficients which are obtainable with conventional lenses utilizing very small apertures and gaps, are obtainable by increasing only the excitation current, even if the gaps are large. These same results are also obtainable for other types of aberration coefficients, such as off-axial astigmatism and comma aberration. Furthermore, the eccentricity of the aperture diameter and the inclination of the aperture are not considered, compared with the conventional objective lens, since the shape of the field distribution that affects the electron beam passing through the aperture of the poles is chiefly dependent upon the geometry of the lower pole piece top.

We have, therefore, confirmed from our investigation with our novel objective lens pole pieces that the smallest possible aberration coefficients depend upon parameters D,, b and 0. Accordingly, these parameters must be taken into account to reduce or eliminate spherical and chromatic aberrations. That is, in general, thechromatic aberration coefficient diminishes by enlarging the conical surface angle and by reducing the ratio of the lower pole face diameter D, to gap 8. With respect to the spherical aberration coefficient, a large surface angle 0 decreases the coefficient for larger excitation current. A small angle will reduce the coefficient at small excitation currents. It is undesirable to make the frustoconical surface angle too small, since the curve of the chromatic aberration coefficient rises. Also, if the conical surface angle is made too large, the concentration of the magnetic field near the top of the lower pole piece weakens, resulting in an environment of large currents. Consequently, best results are obtained at frustoconical angles 0 between 40 and 60. When the gap between the poles is made very large, the concentration of the magnetic field of the lower pole becomes weak and the excitation current must be increased. Moreover, we have confirmed that it is preferable to make the aperture of the lower pole piece very small, since this effects the maximum field. This is usually made less than one-half the lower pole face diameter D,.

An alternative embodiment of our invention is shown in FIG. 4. The frustoconical surface of pole piece 61 has two angles, 0, and 0 with respect to the lens axis. This cone-shaped lens has a magnetic field distribution concentrated at the top of the lower pole piece which is greater than the concentration for the pole pieces shown in FIGS. 1, 2 and 3. Because of this concentration, a much smaller aberration coefiicient can be obtained than in the lens of FIG. 1 from the same amount of excitation current.

By employing the pole pieces of the present invention, both spherical and chromatic aberrations can be reduced without the requirement of large excitation current. Furthermore, diminution of other aberrations such as astigmatism based on the eccentricity of the poles is obtained since the shape of the field distribution depends almost entirely upon the geometry of the lower pole pieces. Moreover, there is very little field disturbance due to the accuracy of the lower pole piece surface, since the field distribution depends largely upon the conical surface magnetic charge.

As described herein above, we have experimented at kv. accelerating voltage, but it may be possible to obtain similar results in the case of different accelerating voltages.

While we have shown and described preferred embodiments of our invention, it is to be understood that it may be otherwise embodied within the scope of the appended claims.

We claim:

' 1. In a magnetic electron objective lens having upper and lower pole pieces spaced apart by a distance S, each of said pole pieces having apertures having diameters b and 17 respectively, the apertures being aligned along a common axis, and said lower pole piece having a pole face diameter D, normal to the axis, the improvement comprising:

A. said lower pole piece being formed with a continuous sidewall having at least one conical taper between 40 and 60 with respect to said axis;

B. the diameter b of the aperture of the pole piece and the diameter D of the lower pole face being equal to or less than the distance S between said pole pieces;

C. the diameter D, of the lower pole piece being equal to or greater than the diameter b of the aperture of the upper pole piece;and

D. means for exciting said pole pieces greater than that required for saturation.

2. The improvement set forth in claim 1 wherein the conical taper forms at least two angles with said axis, the angle nearest the pole face being greater than the other angle.

3. The improvement set forth in claim 1 wherein the distance S between said pole pieces is between 4 and 20mm. and the diameter of the lower pole face is within 6mm. 

1. In a magnetic electron objective lens having upper and lower pole pieces spaced apart by a distance S, each of said pole pieces having apertures having diameters b1 and b2 respectively, the apertures being aligned along a common axis, and said lower pole piece having a pole face diameter D2 normal to the axis, the improvement comprising: A. said lower pole piece being formed with a continuous sidewall having at least one conical taper between 40* and 60* with respect to said axis; B. the diameter b1 of the aperture of the pole piece and the diameter D2 of the lower pole face being equal to or less than the distance S between said pole pieces; C. the diameter D2 of the lower pole piece being equal to or greater than the diameter b1 of the aperture of the upper pole piece; and D. means for exciting said pole pieces greater than that required for saturation.
 2. The improvement set forth in claim 1 wherein the conical taper forms at least two angles with said axis, the angle nearest the pole face being greater than the other angle.
 3. The improvement set forth in claim 1 wherein the distance S between said pole pieces is between 4 and 20mm. and the diameter of the lower pole face is within 6mm. 